The present invention relates generally to techniques for analyzing the performance of a digital data transmission path, and more particularly to a technique for analyzing a digital data transmission path by transmitting a known test signal, receiving the resulting output signal and correlating the received signal with the test signal to determine the transfer function of the digital data transmission path.
GB Patent 11 60 271 discloses a process for detecting the impulse response of a digital data transmission path to be examined, in which a digital random signal is transmitted as a test signal on the input side of the path. An output signal received on the output side of the transmission path in response to the test signal is cross-correlated for purposes of evaluation with the test signal, which test signal is delayed as needed. From the cross-correlation, a function is obtained, which corresponds to the transfer function of the data transmission path to be examined.
If the--generally at first unknown--transfer function of the data transmission path to be examined exhibits non-linearities, then cumulative and differential frequencies occur in the received output signal that lie within the spectrum of the output signal caused by the frequencies of the spectral lines of the emitted test signal. Thus, these spectral components caused by non-linearities cannot be separated from the spectral components of the output signal describing the transfer function. This means that the thus determined transfer function conforms with the actual transfer function of the transmission path to be examined, only if the transmission path is linear, while with increasing non-linearity, there is a considerable deviation between the measured and the actual transfer function.
In the case of a process disclosed by AT-B-380 960 for selective attenuation-transmission measurements in low-frequency transmission systems, the system is fed on the input side with a test signal controlled by a clock-pulse generator and having equidistant spectral lines of the same amplitude of a random-noise generator. The filters of a clock-pulse-generator-controlled matched filter are adjusted to the spectral lines of the test signal. The attenuation ratio can be inferred from the signal level ratio between the test signal and the output signal. In this system, it is not possible to detect and eliminate distortions caused by non-linearities on the system.
A process for determining the transmission properties of an electrical line is disclosed by the non-pre-published WO 92/17949, in the case of which a binary, bipolar random-sequence signal having the crest factor 1 is used as a test signal. Possibilities for determining non-linear behavior of the line are not addressed.
German A1-26 37 775 describes a process for assessing the transfer function of a system, wherein the system is excited on the input side by short-period, transient, broad-band noise signals. As a result of non-linearities of the system, distortions contained in the output signals are eliminated by generating the average value of the output signals from a plurality of individual measurements. It is, thus, possible to estimate the transfer function within a given range of accuracy; the known process can neither be used to detect nor to qualitatively assess a non-linear performance of a system.
The present invention is directed to the problem of developing a process to reliably and simply detect the non-linear behavior of a digital data transmission path to be examined and, from that, to assess, as needed, the quality of a transfer function to be determined.